The present invention relates to a radio-wave arrival-direction estimating apparatus employing an array antenna, and a directivity variable transceiver for varying antenna directivity based on an estimation result from the estimating apparatus.
An arrival direction of radio wave is conventionally estimated accurately in a method such as Multiple Signal Classification (MUSIC) method, using an array antenna comprising a plurality of antenna elements. The MUSIC method is disposed in R. O. Schmidt, xe2x80x9cMultiple Emitter Location and Signal Parameter Estimationxe2x80x9d, Institute of Electrical and Electronics Engineers (IEEE) Trans., AP-34, pp. 276-280 (1986). This method includes an algorism for accurately estimating a direction of a plurality of incident waves with the same frequency band.
In this method, M ( greater than 1) antenna elements receive signals, and a receiving unit connected to each antenna element converts the frequency of each of the received signals, detects a phase of it, and demodulates the received signal to a signal comprising orthogonal I and Q signals. An analog/digital converter (A/D converter) converts the demodulated signal to sampling data and outputs the data to a direction estimating processor. The direction estimating processor estimates a direction of the incident waves using the sampling data by the MUSIC method. In other words, using sampling data x1(k), x2 (k), . . . , xM (k) at sampling time kxcex94T obtained by respective antenna elements, a correlation matrix calculation unit creates receiving vector x (k) written as
x(k)=[x1(k)x2(k) . . . xM(k)]Txe2x80x83xe2x80x83(Equation 1), 
where T shows transposition of a vector, xcex94T is a sampling interval, and k is a natural number. The correlation matrix calculation unit, using receiving vectors x(k) for k=1 to N, further finds Mxc3x97M correlation matrix R written as                               R          =                                    ∑                              k                =                1                            N                        ⁢                                          x                ⁡                                  (                  k                  )                                            ⁢                                                                    x                    ⁡                                          (                      k                      )                                                        H                                /                N                                                    ,                            (                  Equation          ⁢                      xe2x80x83                    ⁢          2                )            
where H shows complex conjugate transposition of a vector.
The calculation unit calculates eigenvalues xcex1xe2x88x92xcexM of correlation matrix R in the descending order, and eigenvactors e1xe2x88x92eM corresponding to eigenvalues xcex1xe2x88x92xcexM.
Next, the calculation unit calculates an evaluation value of an arrival-angle evaluation function, assuming number of the incident waves is S, and using noise spatial eigenmatrix EN=[eS+1, . . . , eM] and a feature that signal eigenvector space ES=[e1, . . . eS] and EN are orthogonal to each other. This EN is formed with (Mxe2x88x92S) eigenvactors, namely column vectors, belonging to a noise eigenvactor space having the relation written as
xcex1xe2x89xa7xcex2xe2x89xa7 . . . xe2x89xa7xcfx80S greater than xcexS+1=xcexS+2= . . . =xcexMxe2x80x83xe2x80x83(Equation 3) 
and ES is formed with eigenvactors e1xe2x88x92eS. In other words, arrival-angle evaluation function F(xcex8) for evaluating orthogonality between EN and ES is defined by                                           F            ⁡                          (              θ              )                                =                      1                                                            a                  H                                ⁡                                  (                  θ                  )                                            ⁢                              E                N                            ⁢                              E                N                H                            ⁢                              a                ⁡                                  (                  θ                  )                                                                    ,                            (                  Equation          ⁢                      xe2x80x83                    ⁢          4                )            
where a(xcex8) is a complex response (hereinafter called a steering vector) of the array antenna as a function of azimuth xcex8. Azimuth xcex8 varies in a predetermined angle range. When azimuth xcex8 equals to the arrival angle, ideally, arrival-angle evaluation function F(xcex8) is infinite. A resultant peak direction of F(xcex8) from calculation for the varied xcex8 is set to be the arrival-angle evaluation value of the incident waves.
Number S of incident waves is generally unknown, so that the number is determined based on an eigenvalue distribution and number-of-signal determination criteria. The criteria is described in M. Wax and T. Kailath, xe2x80x9cDetection of Signals by Information Theoretic Criteriaxe2x80x9d, IEEE Trans. On Acoustics, Speech and Signal Processing, Vol. ASSP 33 (2), pp. 387-392, February (1985).
The radio-wave arrival-direction estimating apparatus employing the MUSIC method discussed above estimates an arrival direction accurately by signal processing, using an algorithm of deriving engenvalue of a correlation matrix of array received signals. In such an estimating apparatus, correlation between waves generated by reflection on the ground or a building increases when a relative delay time between these waves is shorter than a symbol length. In this case, correlation matrix R degrades, and therefore the incident waves cannot be precisely separated.
For preventing the degradation, a spatial smoothing technique is proposed. This spatial smoothing technique is described in Pillai et al, xe2x80x9cForward/Backward Spatial Smoothing Techniques for Coherent Signal Identificationxe2x80x9d, IEEE Trans. On Acoustics, Speech and Signal Processing, Vol. 37, No. 1, 1989. The example has estimated the arrival direction using spatial samples from the array antenna; however the MUSIC method can be similarly applied to a signal sampled every frequency and the delay time of the received waves can be estimated at high resolution.
The estimation accuracy of the arrival direction in the MUSIC method depends on variation step xcex94xcex8 of xcex8 in the arrival-angle evaluation function (Eq.4). When xcex94xcex8 increases, a calculation amount in the entire variation range of xcex8 decreases, but the peak direction of the arrival-angle evaluation function cannot accurately detect the peak direction and the accuracy decreases. When xcex94xcex8 decreases, the peak direction of the arrival-angle evaluation function can be accurately detected, but a calculation amount in the entire variation range of xcex8 increases.
It is an object of the present invention to provide a radio-wave arrival-direction estimating apparatus allowing reduction of a total calculation amount for an arrival-angle evaluation function without causing accuracy degradation of the arrival direction. It is another object of the present invention to provide a directivity variable transceiver for improving transmitting and receiving quality by controlling antenna directivity.
In the present invention, product of a noise spatial eigenmatrix and a conjugated and transposed matrix of it is a product of an upper or lower triangular matrix. Therefore, the calculation amount for the arrival-angle evaluation function can be reduced in the entire angle range for the estimation of the arrival direction. The arrival angle evaluation using the arrival-angle evaluation function that has a heavy calculation load can be significantly reduced during angle sweeping in the MUSIC method. Processing of the arrival-direction estimating apparatus can be speeded or an apparatus structure can be simplified.